UM1557 datasheet(23/111 Pages) STMICROELECTRONICS

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UM1557

Analogue current sense

Doc ID 023521 Rev 2

23/111

datasheet by the parameter VSENSE “maximum analog sense output voltage” (5 V minimum

@ 8 V<Vcc <36 V; -40°C < Tj < 150 °C).

Example 1 – VSENSE saturation (8.5 V typ.):

VND5T035AK with RSENSE selected in order to have VSENSE=2 V @ Iout = 3 A

Considering (for sake of simplicity) K2 @ 3 A = 2870 (typical value)  ISENSE = ~1 mA

 R

SENSE = ~1.9 k

Assuming a typical VSENSE saturation of 8.5 V  maximum ISENSE = 4.5 mA to

maintain linearity

 maximum I

OUT = 12.8 A.

In other words, with the selected RSENSE any load current higher than 12.8 A produces

the same VSENSE (see Figure 17).

Figure 17.

VSENSE vs IOUT

On the other hand, care must be taken to prevent the P channel MOSFET M1 from

saturation, causing the ISENSE to again be disproportional with IOUT. This normally happens

when the maximum current that M1 is able to supply is reached (~14 mA).

This value is consistent with the current sense operating range and current limitation value.

Example 2 – ISENSE saturation (~14 mA):

VND5T035AK with RSENSE selected in order to have VSENSE=1.5 V @ Iout = 10 A

Considering (for sake of simplicity) K3 @ 10 A = 2895 (typical value) 

ISENSE =3.45 mA  RSENSE = 430 Ω

Assuming a typical ISENSE saturation of 14 mA and K to remain approx. 2895 for

IOUT > 10 A, the maximum load current which can be detected is

MAX

SENSE

OUT

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UM1557 Datasheet(PDF) 23 Page - STMicroelectronics